In a gas turbine the most critical temperature is located just downstream of the combustion chamber where excessive temperatures can result in damage or destruction of first-stage turbine blades. Therefore, temperatures must be limited by reducing the amount of fuel entering the combustor if the temperature becomes dangerously high. Because of nonuniform distribution of the temperature-field downstream of the combustion chamber, measurement in this region is not, in general, an accurate measure of blade temperature at the inlet to the first-stage. In addition, it is difficult to have a high-speed temperature measurement near the upstream region of the first-stage inlet. This is mainly due to practicality reasons of tight volume configurations and inaccessibility near the combustion chamber sections to the first-stage inlet. Also, because of an unfriendly environment, the thermocouple transducers require shielding and protective thermowells which affect transducer response time and accuracy.
The present control strategy makes use of readings downstream of the gas generator turbine, where measurement is more reliable because of lower temperatures. Based on rules of thumb, testing, and empirical correlation, the exhaust temperature value at which blade damage is reached is tabulated for different operating conditions of the gas turbine. Typically, the exhaust temperature limiting value is tabulated as a function of air compressor discharge pressure (CDP). In the fuel control system a safety margin is defined to keep the exhaust temperature safely below the exhaust temperature limiting value specified for a given load on the gas turbine.
As a result of the preceding compounding factors, the safety margin defined in the fuel control system (to keep the exhaust temperature at a safe value in order to prevent blade damage) is very conservative. This results in driver limitations leading to production losses, and the inability to operate the gas turbine at higher first-stage inlet temperatures for higher operating efficiency. For the above reasons, there is a need to accurately control and regulate fuel intake during dangerous transients for two- and three-shaft gas turbines.